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Also default these options to `null` for state version ≥ 19.09. Fixes #811 Suggested-by: Sean Marshallsay <srm.1708@gmail.com>
This makes the programs.firefox.package option take a pre-wrapped Firefox package as value if state version is set to "19.09" or later. This should make the Firefox module work with a wider range of Firefox packages.
Also prepares for 19.09.